The present invention relates to a parison for stretch molding which is preliminarily formed for blow molding of vessels such as plastic bottles and so on, and particularly to a closed-end parison.
Plastic bottles such as PET vessels (vessels made of polyethylene terephthalate resin) have been generally molded by preliminarily forming a parison 100 as shown in FIG. 11 and blow molding in which the parison 100 is expanded by blowing compressed air therein to form as a final product the hollow vessel B shown in FIG. 12.
The parison 100 has a closed-end cylindrical form having a cylindrical side wall 101 which forms the barrel portion B1 of the vessel B and a bottom wall 102 which forms the bottom portion B2 of the vessel B and is formed by injection molding. The thickness of each of the side wall 101 and the bottom wall 102 is so determined that the thickness of the vessel B as a final product is a required dimension. In other words, since the vicinity of the bottom wall 102 of the parison 100 becomes thin as a result of stretch molding during blow molding, the whole of the bottom wall 102 is made thick, as shown in FIG. 13. Therefore, an excessive amount of a material is present at the bottom of the conventional container.
However, in such a conventional container, since the thickness of the whole of the bottom wall 102 of the parison 100 is large, the cost of materials is large. In addition, since the large thickness deteriorates the cooling efficiency of the resin of the parison 100 during injection molding, the molding of the parison 100 takes a long time, and the efficiency of the production is poor, resulting in the presence of a limiting factor in the mass-production of the vessel.
If the thickness of the bottom wall 102 is reduced in order to solve the aforementioned problems, as shown in FIG. 14, although the amount of the material used is less and the cooling efficiency is increased, portions B2' in the vicinity of the bottom B2 of the vessel B becomes thin after blow molding, as shown in FIG. 16, resulting in a reduction in strength and the danger of easy damage due to falling of the vessel B, etc.
When the state of the bottom wall 102 of the parison stretched during blow molding was investigated, it was found that since a portion near the center 0' of the bottom wall 102 was hardly stretched, as shown in FIG. 15, a portion near the center 0' of the bottom B2 of the vessel B was thus made brittle and the area around of the center became thin, resulting in a decrease in strength.